The processing and disposal of waste material is an important area of environmental management. Generally speaking waste material is either burnt or buried at landfill sites. Some waste is not suitable for disposal by burning and is thus buried (landfill) or simply spread over an area of land (landfarm). However, some waste material is not suitable for such disposal methods.
Oily sludge is an example of a waste material that cannot suitably be disposed of by burning. Also, because of the fluid nature of oily sludge it can not be simply buried at landfill sites either. This is because, under present legislation, waste material must be buried in a solid form. The practice of landfarming is also undesirable for environmental reasons.
The appearance of said sludge is that of thick mud and it is usually unsuitable to be further dewatered by pressing (e.g. belt press, chamber press) or centrifuging to the required dryness and shear stability for landfill; especially without chemically treating the sludge.
Under impending legislation waste, nominally solid, having an oil content of greater than certain specified values (i.e. 0.1%, 5%, 6%) will result in the material being classified as hazardous. The specified oil content of a particular batch of waste will determine the eventual route of disposal available for the waste.
Waste with an oil content of between above 0.1% and 5% can be disposed of at hazardous waste sites and non-hazardous waste sites, provided they have special cells. Waste having an oil content of 6% and over must be disposed of at a hazardous waste site. This distinction can create significant cost differences between batches of waste having an oil content of 5% and waste having an oil content of 6%, mainly due to transport costs and landfill site costs.
In view of the legal, financial and environmental requirements, it is obviously more preferable to produce waste having an oil content of less that 6%, and yet more preferably 0.1% or below. In view of the above situation, at present oily sludge is processed to ensure that it is in the necessary state for disposal.
The pretreatment processes currently in use for oily sludge, which include the solidification of the sludge using absorbents, are both expensive and wasteful as they may not involve the reclamation of any of the hydrocarbons held within the sludge. Other techniques such as thermal drying and desorption of hydrocarbons may be used but these are expensive and energy intensive processes.
In light of the state of the art there is a need for a method of processing oily sludge to transform it into a form that can be more suitably disposed of. The processing method of the present invention is targeted mainly at the treatment of oily sludge that arises from refineries and waste oil recovery/treatment plants, where waste oil and oily sludge are recovered to make fuel oil from industrial waste oil and garage forecourt recycling schemes.
It is generally accepted that plate and frame (or chamber) presses are not suitable for processing oily sludge due to the likelihood of blinding. As a result, belt presses are usually used in the processing of oily waste. Typically, a belt press can only achieve a maximum of 20% solids and a minimum of 10% oil, which as a ratio of 2:1. There is therefore a need for a method of achieving waste with a much lower solid content to oil content ratio.